Datasheet

TPS24720

SLVSAL1C –MARCH 2011–REVISED SEPTEMBER 2013

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Figure 29. Inrush Mode at Hot-Swap Circuit Insertion

INRUSH OPERATION

After TPS24720 initialization is complete (as described in the Board Plug-In section) and EN is active, GATE is

enabled (V

GATE

starts increasing). When V

GATE

reaches the MOSFET M1 gate threshold, a current flows into the

downstream bulk storage capacitors. When this current exceeds the limit set by the power-limit engine, the gate

of the MOSFET is regulated by a feedback loop to make the MOSFET current rise in a controlled manner. This

not only limits the capacitor-charging inrush current but it also limits the power dissipation of the MOSFET to safe

levels. A more complete explanation of the power-limiting scheme is given in the section entitled Action of the

Constant Power Engine. When GATE is enabled, the TIMER pin begins to charge the timing capacitor C

with a

current of approximately 10 µA. The TIMER pin continues to charge C

until V

(GATE – VCC)

reaches the timer

activation voltage (5.9 V for V

VCC

= 12 V). The TIMER then begins to discharge C

with a current of

approximately 10 µA. This indicates that the inrush mode is finished. If the TIMER exceeds its upper threshold of

1.35 V before V

(GATE – VCC)

reaches the timer activation voltage, the GATE pin is pulled to GND and the hot-swap

circuit enters either latch mode or auto-retry mode, depending upon the status of the LATCH pin (see LATCH in

Detailed Pin Descriptions).

The power limit feature is disabled once the inrush operation is finished and the hotswap circuit becomes a

circuit breaker. The TPS24720 will turn off the MOSFET M1 after a fault timer period once the load exceeds the

current limit threshold.

ACTION OF THE CONSTANT-POWER ENGINE

Figure 30 illustrates the operation of the constant-power engine during start-up. The circuit used to generate the

waveforms of Figure 30 was programmed to a power limit of 29.3 W by means of the resistor connected between

PROG and GND. At the moment current begins to flow through the MOSFET, a voltage of 12 V appears across

it (input voltage V

VCC

= 12 V), and the constant-power engine therefore allows a current of 2.44 A (equal to 29.3

W divided by 12 V) to flow. This current increases in inverse ratio as the drain-to-source voltage diminishes, so

as to maintain a constant dissipation of 29.3 W. The constant-power engine adjusts the current by altering the

reference signal fed to the current limit amplifier. The lower part of Figure 31 shows the measured power

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